I just deleted steamapps\common\left 4 dead 2\bin\mss32.dll and that worked fine!

(I assume it's a primary location for the game to load files from, so deleting individual files makes the game fall-back on the originals in the VPK files. Also it means that a game-cache check wouldn't detect it.)

I have the same problem too, I deleted the bin folder, verified the game cach files, four files failed to validate, then I verified them again, then all the files validated fine, and I still get the same error message.

Hello, guys!I have the same problem. Tried all above several times. Re-download game and steam - doesnt help. Before everything works fine. Then I had a notification about missing fail, but game works. Now it doesnt.

Hello, guys!I have the same problem. Tried all above several times. Re-download game and steam - doesnt help. Before everything works fine. Then I had a notification about missing fail, but game works. Now it doesnt.

I switched antivirus - doesnt help.

Is it possible - someone has diffirent decision?

You just need to google mss32.dll and redownload that file in l4d game/bin directory. Choose to replace it with new one and you're good to go.

No need to delete or replace mss32.dll, or delete the whole 'bin' folder. What fixed this error for me is to delete mssvoice.asi in \Steam\steamapps\common\Left 4 Dead 2\bin. I left mss32.dll untouched.